The invention relates to techniques for recognizing hand-drawn graphic symbols, and particularly to a technique utilizing previously vectorized data produced in response to electronic scanning of a drawing.
Many technical drawings of mechanical systems, electrical systems, and the like include large numbers of a wide variety of shapes or symbols, such as logic element symbols, electric circuit element symbols, and mechanical element symbols. Such symbols usually do not stand alone from other parts of the drawing, but are directly connected by lines to other symbols or parts of the drawing. In the development of a relatively low cost automatic drawing recognition system that can rapidly convert engineering drawings, hand-drawn technical diagrams, and the like to digital data that can be input to various CAD (computer aided design) systems, it would be very desirable to be able to automatically recognize such symbols in order to allow the system to efficiently refer to them by assigned codes which can be used to fetch corresponding symbols from stored symbol libraries, and thereby avoid the need to store and manipulate a much larger amount of digital data that would be completely descriptive of such symbols.
Quite a variety of work has been previously published in the field of character recognition. Prior character recognition systems involve statistical feature extraction and recognition techniques applied to relatively small, "isolated" characters that are not connected by dark lines to any other character or graphic structure. The prior character recognition techniques are not suitable for recognizing graphic symbols which are connected by dark lines to other symbols or graphic structures. Furthermore, conventional character recognition techniques require a great deal of computing time and memory space.
A prior technique referred to as a "blackboard" technique has been utilized in language recognition software systems. See "The Hearsay-II Speech-Understanding System: Integrating Knowledge to Resolve Uncertainty", by L. D. Erman, F. Hayes-Roth, V. Lesser and D. Reddy, "Computing Surveys 12", pages 213-253, 1980. Software "blackboards" are utilized as repositories for intermediate results to form labelled structures from sequentially received input data. The blackboard technique also has been applied to hypothesis formation techniques in building of so-called expert systems. For example, see "Two Theoretical Issues Concerning Expert Systems", IEEE International Conference on Systems, Man and Cybernetics, by Findler, N. V., and P. Bhaskaran, pp. 236-240, 1985.
Thus, there is an unmet need for an improved technique for recognizing graphic symbols which are directly connected to other objects on a drawing.